Piezoelectric sensor

ABSTRACT

A piezoelectric sensor includes a carrier, a piezoelectric measurement sensing element arranged on the carrier, a covering layer covering the measurement sensing element and an electronic evaluation unit. The measurement sensing element is formed by a piezoelectric layer. The carrier has a first contact layer electrically connected to the piezoelectric layer and the covering layer has a second contact layer electrically connected to the piezoelectric layer. The electronic evaluation unit is able to determine a mechanical loading of the piezoelectric layer by evaluating the difference of electrical potential between the first contact layer and the second contact layer.

RELATED APPLICATIONS

[0001] This application claims the benefit of PCT InternationalApplication Serial No. PCT/DE01/02362, filed Jun. 29, 2001 which claimsthe benefits of German Utility Model Application Serial No. 100 31793.6, filed Jul. 4, 2000.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The invention relates to piezoelectric sensors.

[0004] 2. Description of the Related Art

[0005] A piezoelectric sensor is known from German Patent 28 431 938. Inthis sensor the charge transfer of a piezoelectric film, which isconverted into the desired measurement signal via an external electronicevaluation unit, is used as the measurement value. These sensors havethe disadvantage that the length of the paths to the electronicevaluation unit are limited and a further component must be arrangedremotely with the electronic evaluation unit.

[0006] Sensors of the above-mentioned type are known in addition asimpact sensors having a seismic mass, the seismic mass being pressedagainst the piezoelectric layer as a result of an impact and theacceleration caused thereby. This pressure on the piezoelectric layercauses, in turn, a charge transfer which can be picked up and evaluatedby means of contacts on each side of the surface of the piezoelectriclayer. A sensor of this type is described by Gevatter in “Handbuch derMess- und Automatisierungstechnik”, VDI Verlag 1998.

[0007] These sensors with a seismic mass have, in addition to thedisadvantages of the above-mentioned film-type sensors, the disadvantagethat the seismic mass must be excited. Furthermore, because of themovable guidance system of the seismic mass, a comparatively complex andcostly mechanism must be provided which, because of the moving parts,leads to high production costs and, in addition, to a higher risk offailure. Finally, construction of very sensitive sensors is possibleonly at disproportionate cost, as measurement is only possible if theseismic mass is excited by the weak signal. For this reason a sensor ofthis kind can hardly be used, for example, as a vibration meter.

[0008] Hence, those skilled in the art have recognized a need for asensor which can be manufactured at low cost, is easily installed andcan measure economically and in a simple manner even weak signals, suchas vibrations or material deformations, by utilizing the piezoelectriceffect. The invention fulfills these needs and others.

SUMMARY OF THE INVENTION

[0009] Briefly, and in general terms, the invention is directed to apiezoelectric sensor including a carrier, a piezoelectric measurementsensing element arranged on the carrier, a covering layer covering themeasurement sensing element and an electronic evaluation unit. Themeasurement sensing element is formed by a piezoelectric layer. Thecarrier has a first contact layer electrically connected to thepiezoelectric layer, the covering layer has a second contact layerelectrically connected to the piezoelectric layer and the electronicevaluation unit is able to determine a mechanical loading of thepiezoelectric layer by evaluating the difference of electrical potentialbetween the first contact layer and the second contact layer.

[0010] These and other aspects and advantages of the invention willbecome apparent from the following detailed description and theaccompanying drawings which illustrate by way of example the features ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 is a side view of a sensor according to the invention;

[0012]FIG. 2 is a top view of the sensor illustrated in FIG. 1; and

[0013]FIG. 3 shows a networking of a plurality of sensors for monitoringan industrial manufacturing process.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] Referring now to the drawings, wherein the reference numeralsdenote like or corresponding parts throughout the figures, andparticularly to FIG. 1, there is shown a piezoelectric sensor formed bya carrier 2 and a film-like piezoelectric layer 1 arranged thereon. Thepiezoelectric layer 1 is covered by a covering layer 3, both thecovering layer 3 and the carrier 2 are configured to be electricallyconductive to the piezoelectric layer 1. For this purpose the carrier 2is provided at least on its side facing towards the piezoelectric layer1 with a first contact layer 5. The covering layer 3 is provided with asecond contact layer 6. Both layers, the first contact layer 5 and thesecond contact layer 6, can be produced on the respective component bymeans of vacuum evaporation; alternatively thin metal or precious metalfoils can be bonded thereto.

[0015] In the front area, the covering layer 3 is connected to thecarrier 2, the conductive second contact layer 6 is connected to aninternal electronic evaluation unit 4, also arranged on the carrier, bymeans of conductive tracks. The first contact layer 5 is connected tothe electronic evaluation unit 4 likewise by means of conductive tracks.This electronic evaluation unit 4 can carry out the complete evaluationof the measurement signal or can take over only a part of the signalprocessing and can transmit an intermediate signal to an external signalprocessing unit 8 (not shown). For this purpose the connecting cable inthe exemplary case illustrated is provided with a standardized interface9 which makes possible simple connection of the sensor to a field bussystem.

[0016] For reasons of clarity dimensions are not reproduced to scale inthe figures. In practice the piezoelectric layer 1, like the firstcontact layer 5 and the second contact layer 6, will be significantlythinner. In the embodiment shown the sensor is surrounded by a housing7, illustrated here as a hollow housing. However, the housing 7 ispreferably injection molded around the functional components so that thelatter are enclosed in an air-free manner. The housing 1 can alsoenclose the carrier 2 or can be closed laterally by the latter. In bothcases the housing 7 must be so designed that twisting of thepiezoelectric layer 1 analogously to the vibration or deformation of thecomponent on which the sensor is arranged remains possible.

[0017]FIG. 2 shows the sensor illustrated in FIG. 1 in a top view,partially in cross-section. In the right-hand area the piezoelectriclayer 1 with the covering layer 3 is arranged in a sandwich structure onthe carrier 2, the first contact layer 5 and the second contact layer 6being interposed respectively between the above-mentioned elements. Theinternal electronic evaluation unit is represented schematically hereand consists of a miniature integrated circuit having the usualelectronic components for signal processing, which integrated circuitcan be formed by the left-hand part of the carrier 2 or can be placedthereon. The covering layer 3, for example a thin silver foil, isconnected in the area facing towards the electronic evaluation unit 4 toa contact element which in turn is connected to the electronicevaluation unit 4 via conventional conductive tracks. The bonding to thefirst contact layer 5 located below is effected in a similar manner.

[0018] In one embodiment of the invention the evaluation of themeasurement signal is not carried out exclusively via the internalelectronic evaluation unit 4. In this case the interface 9 is used forconnection to an external electronic evaluation unit 8 which takes overfurther signal processing. By this means triggering and initializing ofthe sensor can be carried out. In particular in the case of very weaksignals which can occur, for example, when the sensor is used as avibration sensor for detecting disturbance signals, the cost requiredfor miniaturization can be reduced by means of an external electronicevaluation unit 8.

[0019]FIG. 3 shows an exemplary application of the invention in whichdefects and damage to pumps can be detected by monitoring thesolid-borne sound in the pump housing. In this case normal vibrationsare manifested by pump noises which, in case of damage, for example,damage to a pump wheel, are changed in frequency or amplitude. For thispurpose the sensor according to the invention first measures the“normal” pump noise and records it as operating noise. In case ofchanges to this noise the change can be filtered out by frequencyanalysis and interpreted as a defect or as a normal change caused byoperating conditions.

[0020] In the application illustrated in FIG. 3 the individual sensorsare interconnected via a field bus which makes possible connection ofthe interfaces 9 to the external electronic evaluation unit 8. For thispurpose a common evaluation and control unit is connected to thesensors, a display and operating element being interposed. If defectsare detected the required measures can be taken by means of conventionalcontrol systems.

[0021] The sensor together with the evaluation electronics is configuredto have the thickness of a film, the piezoelectric layer having athickness of less than 1 mm and the evaluation electronics together withthe measurement sensing element being arranged on the film-like carrierwhich is manufactured from an elastic material which damps vibrations toonly a slight degree.

[0022] Through the configuration of the sensor according to theinvention the seismic mass is dispensed with and a thin, piezoelectriclayer arranged on the elastically deformable carrier is used as themeasurement sensing element, the deformations of which carrier itconverts into a measurement signal. The evaluation electronics arearranged on the carrier so that transmission of the weak piezoelectricmeasurement signal from the film to the evaluation electronic unit overa relatively long distance is unnecessary. In this way the cost ofmanufacturing and installing the sensor can be considerably reduced andthe sensor can be provided for the first time as a piezoelectric sensorfor entirely new applications.

[0023] The assemblies of the new type of sensor are the flexible carrierand the covering layer, together with the piezoelectric layer arrangedtherebetween. The covering layer and the carrier are so configured thatthe charge transfer within the piezoelectric layer as a result of adeformation can be picked up by the carrier and covering layer. Thedeformation of the piezoelectric layer can be both a twisting about anyaxis and a pressure acting in a vertical direction on the layer itself.

[0024] All the components of the sensor are sufficiently flexible forthe sensor, which can be bonded to a surface, to be able to convertvibrations present in the component carrying the sensor as a result ofsolid-borne sound, into vibrations of the piezoelectric layer. This inturn produces a charge layer which changes over time as a function ofthe amplitude and duration of the vibration, which charge layer can bepicked up via the carrier and the covering layer and converted into thedesired signal by the evaluation electronics.

[0025] A sensor constructed in this manner can be used in a multiplicityof applications and, because it can be manufactured at especially lowcost, can often be integrated as an additional link in a control chain.For example, in a preferred embodiment of the sensor, the sensor can beused in the context of triggering an airbag of a motor vehicle, whichairbags are at present triggered almost exclusively by impact sensors.In particular, in accidents in which the vehicle is first subjected toan impact but has not yet suffered the accident, for example whencolliding with a safety barrier and subsequently skidding, the airbag isfrequently triggered by the first impact and, because it collapses againafter a few fractions of a second, is no longer available during theactual crash of the vehicle. In this situation the sensor according tothe invention can be used in addition to the electronic system and, forexample, can be bonded to the inside of a body panel of the vehicle. Thesensitivity of the sensor can be so adjusted via the evaluationelectronics that a relatively large deformation, in addition to theimpact, must be necessary to trigger the airbag.

[0026] The piezoelectric layer of the sensor is only a few μm thick andis preferably manufactured from a film. Piezoelectric film of this kindis obtainable by the meter, the price for 1 m² of film beingapproximately the same as the price of the piezoelectric sensors withseismic mass used hitherto. A large number of sensors can bemanufactured from 1 m² of film, so that, bearing in mind the fact thatthe remaining components of the sensor involve no significant costs, thesensor according to the invention can be manufactured approximately 10to 50 time more cheaply than the sensors known and used hitherto. Thismakes its use in many applications economic for the first time. Multiplemonitoring by the use of a plurality of sensors also now becomespossible and economic.

[0027] Through the use of the piezoelectric film in almost any desiredshape an impact sensor can be constructed, for example, as protectionagainst pinching. For example, in the case of a roll-up door, a lowerrubber lip can be provided with the measurement sensing element, thecarrier being formed by an outer rubber layer and the covering layer byan inner rubber layer. When the rubber lip makes contact at any pointthe sensor will emit a signal which can be used to switch off the doormechanism permanently or temporarily in case of jamming. This can alsobe used for doors of a public transport vehicle or forelectrically-actuated windows.

[0028] Further exemplary applications of the sensor according to theinvention are the detection of vibrations as a result of machine damage,such as can occur in pump housings, in shaft bearings or in railvehicles. For example, the fracture of a wheel rim which would endangertrain operation can be detected by the additional vibrations occurringas a result of the fracture and a signal can be emitted, or a controlintervention made, in good time before derailment of the train. In thecase of a pump housing, which must normally tolerate a uniform vibrationas a result of splashing of the liquid to be pumped or as a result ofbearing noise, the deviation from the normal frequencies or amplitudesin case of bearing damage can be detected and action can be taken beforemore serious damage occurs. Finally, the sensor can also be used for theconstruction of alarm systems or movement detectors, for example whendisplay windows or motor vehicles are to be protected from maliciousdamage by scratching.

[0029] The carrier is preferably manufactured from a flexible material,for example a flexible plastics material. The first contact layer, whichcan be formed, for example, by a vacuum-evaporated coating of silver,can then be applied to this material. Bonding on of a metal foil or afoil made of another conductive material is also possible.

[0030] In a preferred embodiment of the invention the carrier is dividedinto two areas, the actual sensor being arranged in a sensor area whileat least a part of the evaluation electronics is arranged in an adjacentarea. In the sensor area the first contact layer is applied, whileleaving free an edge portion, to which contact layer the thinpiezoelectric film of the same size can then be bonded. The coveringlayer, which carries the second contact layer in the area of thepiezoelectric film, is applied in turn to this piezoelectric film. Thissecond contact layer can also be applied to the covering layer by vacuumevaporation or can be bonded on as a foil. The first contact layer andthe second contact layer are then connected to the evaluationelectronics, and have in particular a cable connection to correspondingcontacts or are so configured that they have prolongations extending inthe manner of conductive tracks in the direction of the evaluationelectronics and connected to the corresponding contacts.

[0031] In an alternative embodiment of the invention the contact layercan also be arranged inside or below the carrier or the covering layer,in which case the electrical contact to the piezoelectric layer is madevia through-connections.

[0032] Preferred thicknesses of the piezoelectric layer are less than 1mm, in particular a few μm, in particular less than 20 μm, thicknesseseven below 10 μm being possible. In a concrete embodiment of the sensoras a vibration meter a thickness of the piezoelectric layer of, forexample, 6 μm is used. This piezoelectric film is joined to the carrierand the covering layer with inclusion of the first contact layer and thesecond contact layer, the connection being effected in particular bybonding during the manufacturing process.

[0033] The carrier can be manufactured from an electricallynon-conductive material, although it is also possible for the carrieritself to be conductive. In this case it is no longer necessary to applya separate first contact layer and it is sufficient if the carrieritself forms the first contact layer. The same applies in an analogousmanner to the covering layer which, of course, can also be electricallyconductive. The carrier and the covering layer can have a thickness ofless than 1 mm, a material thickness between 120 μm and 160 μm beingespecially preferred. In the case of a concrete embodiment the firstcontact layer and the second contact layer have a thickness of 5 to 50μm, so that a total thickness of the sensor in the area of themeasurement sensing element of less than 300 μm (without housing)results.

[0034] By dispensing with the seismic mass the sensor can therefore bekept very thin, enabling it to be used where it has not hitherto beenpossible to use piezoelectric sensors or sensors of any kind. At thesame time the sensor has good resistance to pressure. For this reason itcan be used as a washer in a screw connection, the pressure sensingelement being clamped by the screw insertion force and the evaluationelectronics being arranged beside the sensor. By means of this sensorthe screw insertion force and an impact load on the screw can bemonitored and, for example, slackening of the screw or inadmissibletightening can be detected at an early stage.

[0035] The possibility of using the sensor as a pressure-loaded impactsensor in a screw connection makes it possible to measure and controlthe sealing force of a packaging machine, in which the two halves of thetool are pressed against one another in the context of vacuum packaging.In this case the sealing force can be measured via measurement of theretention force of the clamping screws and regulated if necessary.Finally, an internal pressure in a chamber, for example, can also bemeasured by means of a sensor of this kind.

[0036] In the case of an extruder, for example, the internal pressure inthe front area of the extruder can be measured by means of theabove-mentioned principle via the retention force of the screws by whichthe orifice cap is fixed to the extruder housing, without the need tointroduce a pressure measuring device into the chamber by means ofcomplex and expensive constructional measures. In the case of aninternal combustion engine a sensor could be arranged in the area of thevalve-actuating cam, which sensor measures the closing pressure of thevalve and therefore the pressure in the combustion chamber. By comparingthe internal pressures in the cylinders a defect can thereby be quicklyand reliably detected without major complexity or cost.

[0037] Through the omission of the seismic mass a very sensitive sensoris produced at low manufacturing cost. A sensor manufactured accordingto the above-mentioned principle can, for example, if bonded to atable-top, detect whether or not speaking is taking place in a room. Itis therefore so sensitive that it can convert the sound wavestransmitted to the table-top and further transmitted therein assolid-borne sound, into a measurement signal. A conventionalpiezoelectric sensor with seismic mass would require a very costlymethod of mounting this mass.

[0038] The first and second contact layers can also be subdivided intosegments to construct a still more sensitive sensor, in which case theevaluation electronics should pick up the potential difference betweeneach two opposed segments of the first and second contact layers. Inthis way, not only can spatial information on the charge transfer withinthe piezoelectric layer be obtained but the risk of failure of thesensor can be reduced, since a plurality of contacts are present and thefailure of one pair of segments will not cause the failure of thesensor. In particular in the case of self-calibration of the sensor, thesensor is so adjusted by the subsequent calibration process in case offailure of one pair of sensors, for example as a result of a brokencable, that adjacent segments can take over the function of the failedpart.

[0039] The sensor is preferably integrated together with the evaluationelectronics in a housing, which housing can be produced especiallyeasily by molding the functional components into a plastics block. Thecarrier can be installed on a further carrier plate which can be mountedat the actual point of application, for example a vehicle body panel ora pump housing. To transmit the vibrations of the housing, which, it inthe case of a vibration meter, are to be measured, either the carrier orthe additional interposed carrier element can be bonded to the housingto be monitored.

[0040] Instead of a rigid housing the housing can also be formed by afilm which encloses the remaining functional parts of the sensor itself.For this purpose, in a manner similar to the technique known from vacuumpackaging, a lower and an upper film can, for example, be provided, thesensor being arranged either completely or partially between thesefilms. After evacuation of the cavity these films are either joined bymeans of a continuous peripheral seam, for example by welding or bymeans of a bonded seam. The sensor manufactured in this way can, forexample, then be bonded to the component to be monitored.

[0041] The evaluation electronics preferably include a programmableamplifier which makes it possible to tune the evaluation electronics tothe vibration to be measured in relation to the background signal. Theevaluation electronics can be completely arranged on the carrier,although it is also possible for the evaluation electronics to bearranged only partially on the carrier and for an external part of theevaluation electronics to be housed in a separate housing. The latter isthen connected to the sensor itself via a digital cable or another cablemeeting a conventional bus standard. Both the evaluation electronics onthe carrier and external evaluation electronics preferably have aninterface for a standard bus system. In this way the sensor according tothe invention can be easily and simply integrated into a control system,or example in the context of a monitored manufacturing process.

[0042] The evaluation electronics preferably include a signal processorand a program memory in which is stored software which controls thesensor and evaluates the measurement or the measured value. By means offrequency analysis of the time behavior of the measured potentialdifference the software detects an unusual frequency change and in thiscase emits a signal. In this way the regular pump noise caused bysplashing and bearing noise in the case of pump monitoring cannottrigger a signal, whereas damage to a bearing shell and an unusualfrequency of the solid-borne sound caused thereby triggers a controlsignal or even a speed reduction or unloading of the pump.

[0043] To be able to perform this function the software must be able todistinguish the usual operating noise from unusual noises. For thispurpose a self-calibrating routine is preferably provided which is runfrom time to time and at the start of operation. For this purpose thesoftware records the change over time of the solid-borne sound during apreset period, this period being initially interpreted as the normalcase. During subsequent measurements the measurement signal is then ineach case compared to the signal measured in the reference period and anunusual deviation is interpreted as a defect or damage. In this case analarm signal can be emitted or an automatic intervention can be made inthe control system of the device monitored. To avoid false alarms orother malfunctions a tolerance limit can be preset which must beexceeded for the fault signal to be emitted. This tolerance limit canlikewise be adjustable.

[0044] By a particular configuration of the evaluation electronics thesensor, which operates in principle as a dynamic sensor, can also beused as a static sensor at least for a certain time period. For thispurpose the electronics include an integrator which integrates themeasured signal and thereby determines the actual state even without achange over time of the signal. By the use of digital evaluationelectronics the value of the magnitude measured can also be permanentlydetermined.

[0045] By means of an additional pressure mass the sensor according tothe invention can also be used as a pressure sensor or an accelerometer.This pressure mass can be formed, for example, by a flat pressure platewhich is mounted in a perpendicularly displaceable manner with respectto the sensor. The sensor can be arranged between a surface of acomponent to be monitored and the pressure plate, whereby anacceleration in both directions can be measured by means of anadditional spring force acting between the pressure plate and thesensor. This additional compressive force can be formed by one or moresprings or by a resilient intermediate layer which is arranged, forexample, between the sensor and the pressure plate and causes acontinuous loading of the sensor.

[0046] When the pressure plate is accelerated either a force in thedirection of the measurement sensing element or an unloading of themeasurement sensing element is obtained, causing a charge transferwithin the piezoelectric layer. The pressure plate can cover the entiresensor or can act only on a partial area thereof. In the case of apossible embodiment of the sensor configured as an accelerometer thehousing can be screwed to a component to be monitored and the pressureplate can have sufficiently large through-holes for it to be fixable tothe sensor by means of the screws used. In this case, of course, thehousing must be so configured that transfer of the compressive forcesfrom the pressure plate to the piezoelectric measurement sensing elementis possible.

[0047] Although, with this extension of the measurement sensing element,a seismic mass is again incorporated the advantageous properties of thesensor according to the invention can nevertheless now be combined withthe additional performance feature of acceleration measurement. Forexample, a very sensitive, low-cost sensor can be constructed accordingto the inventive principle which can also detect an acceleration withouttwisting or other deformation of a component. By contrast, although aconventional sensor with seismic mass could measure acceleration, toachieve the same sensitivity in measuring solid-borne sound it wouldneed to be constructed in a very much more complex and expensive manner,since vibrations can only be measured if the seismic mass is excited. Bymeans of the invention this detour via the utilization of massexcitation can be dispensed with.

[0048] It will be apparent from the foregoing that while particularforms of the invention have been illustrated and described, variousmodifications can be made without departing from the spirit and scope ofthe invention. Accordingly, it is not intended that the invention belimited, except as by the appended claims.

What is claimed is:
 1. A piezoelectric sensor comprising a carrier, apiezoelectric measurement sensing element arranged on the carrier, acovering layer covering the measurement sensing element and anelectronic evaluation unit, the measurement sensing element being formedby a piezoelectric layer, the carrier having a first contact layerelectrically connected to the piezoelectric layer, the covering layerhaving a second contact layer electrically connected to thepiezoelectric layer and the electronic evaluation unit being able todetermine a mechanical loading of the piezoelectric layer by evaluatingthe difference of electric potential between the first contact layer andthe second contact layer, wherein the sensor together with theelectronic evaluation unit is configured to have the thickness of afilm, the piezoelectric layer having a thickness of less than 1 mm andthe electronic evaluation unit being arranged beside the measurementsensing element on the film-like carrier which is manufactured from anelastic material which damps vibrations to only a slight degree.
 2. Thepiezoelectric sensor of claim 1 wherein the carrier and thepiezoelectric layer are joined together in a planarly adhering manner.3. The piezoelectric sensor of claim 2 wherein the thickness of thepiezoelectric layer is less than 10 μm.
 4. The piezoelectric sensor ofclaim 1 wherein the carrier has a carrier layer made of an electricallynon-conductive material which is provided with the first contact layerand on which, beside the piezoelectric layer, the electronic evaluationunit is arranged.
 5. The piezoelectric sensor of claim 1 wherein next tothe electronic evaluation unit a planar sensor area is built up in asandwich structure, the carrier being provided in the sensor area with afirst contact layer, the piezoelectric layer being arranged on the firstcontact layer and electrically connected thereto and the covering layerhaving the conductive second contact layer which faces towards thepiezoelectric layer being arranged on the latter.
 6. The piezoelectricsensor of claim 1 wherein the carrier and/or the covering layer is/areformed by a flexible film the thickness of which is less than 200 μm. 7.The piezoelectric sensor of claim 1 wherein the thickness of the firstcontact layer and the thickness of the second contact layer are in eachcase less than 70 μm.
 8. The piezoelectric sensor of claim 1 wherein ithas a housing containing the other components.
 9. The piezoelectricsensor of claim 1 wherein the housing is manufactured by molding ofplastics material around the sensor.
 10. The piezoelectric sensor ofclaim 1 wherein the housing is formed by an upper and a lower film whichare joined together by a connecting seam surrounding the remainingcomponents of the sensor.
 11. The piezoelectric sensor of claim 1wherein the first contact layer and the second contact layer are formedby thin foils of a metallic material.
 12. The piezoelectric sensor ofclaim 11 wherein the foils of the first contact layer and the secondcontact layer have in each case at least one cable-like prolongationwhich is connected to the electronic evaluation unit for bonding. 13.The piezoelectric sensor of claim 1 wherein the first contact layer andthe second contact layer are subdivided into segments by means ofelectrically insulating gaps and the electronic evaluation unit isconnected to each of the segments in such a way that it can determinethe potential difference between a pair of segments of the first contactlayer and of the second contact layer.
 14. The piezoelectric sensor ofclaim 1 wherein the electronic evaluation unit includes a programmableamplifier.
 15. The piezoelectric sensor of claim 1 wherein theelectronic evaluation unit has an interface for connection to a fieldbus, in particular a CAN bus.
 16. The piezoelectric sensor of claim 1wherein the electronic evaluation unit includes a signal processor and aprogram memory with software stored therein, the software being able todetect an unusual frequency change by means of frequency analysis of themeasured potential difference for measurement of solid-borne sound. 17.The piezoelectric sensor of claim 16 wherein the software includes acalibrating routine which after a preset period identifies the changeover time of the solid-borne sound as the normal case and uses thelatter as the basis for subsequent control measurements, the measuredsignal being compared to that of the normal case and the software beingable to emit a signal in case of deviations above a preset tolerancelimit.
 18. The piezoelectric sensor of claim 1 wherein it has a secondexternal electronic evaluation unit which is connected to the electronicevaluation unit via a digital connection and has an interface forconnection to a field bus system.
 19. The piezoelectric sensor of claim1 wherein it has a pressure plate arranged parallel to the piezoelectriclayer, which pressure plate is guided movably in a directionperpendicular to the piezoelectric layer.
 20. The piezoelectric sensorof claim 19 wherein a preloaded resilient element is arranged betweenthe piezoelectric layer and the pressure plate.